Apparatus for microindentation hardness testing and surface imaging incorporating a multi-plate capacitor system

ABSTRACT

A force, weight or position sensor unit and sensor element incorporated into an apparatus for microindentation hardness testing and surface imaging which allows immediate imaging of the surface subsequent to hardness testing. The sensor uses a multi-capacitor system having drive and pick-up plates mounted on an appropriate suspension system to provide the desired relative motion when a force is applied to the pick-up plate. The output signal is run through a buffer amplifier and synchronously demodulated to produce a signal proportional to force or displacement. The sensor element is mounted on a scanning tunneling microscope base and a sample mounted on the sensor. The force sensor is used for both measuring the applied force during microindentation or micro hardness testing and for imaging before and after the testing to achieve an atomic force microscope type image of the surface topography before and after indentation testing.

RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.08/690,909, filed Aug. 1, 1996, now abandoned, which is a continuationof U.S. application Ser. No. 08/327,979, filed Oct. 24, 1994, now U.S.Pat. No. 5,553,486, which is a continuation-in-part of U.S. applicationSer. No. 08/131,405, filed Oct. 3, 1993, now abandoned.

TECHNICAL FIELD

The present invention relates to apparatus for microindentation hardnesstesting and subsequent surface imaging of the results with highresolution capability. More particularly, it is directed to such devicesincorporating a sensor, including a multi-plate capacitor system.

BACKGROUND OF THE INVENTION

Many applications for precise measurement of force, weight, and relativeposition are known in the art. For example, machine shop tools forprecisely indicating or fabricating holes, channels or other surfacefeatures relative to one another require accurate position ordisplacement measurement. Accurate measurement of displacement orposition on small parts, such as those used in the manufacture ofelectronic components is particularly important.

Measurement of force or weight accurately at minute quantities, alongwith instruments to accomplish such measurements are well known. Straingauge transducers are one industry recognized instrument for suchmeasurements. These instruments can be used in laboratory analysis, suchas micro hardness testing of samples. Furthermore, laboratory scales formeasuring constituent components in minute quantities with highresolution are well known in chemical, biological, drug and medicalfields.

A known limitation to resolution in strain gauge transducers is thesignal to noise ratio of the instrument. Strain gauge transducers havean output of only a few millivolts. It is recognized that the minimalpossible noise level for the strain gauge transducer is set by thethermal noise on the strain gauge resistive element. For example, thecalculated noise for a commercial strain gauge sensor with 350 Ohmresistance is 2.4 nV at 1 Hz bandwidth.

In more recent years, the development of scanned-probe microscopes hascreated a need for higher resolution measurement of force and positionat minute levels. As disclosed by Wickramasinghe in "Scanned-ProbeMicroscopes", Scientific American, October, 1989, pp. 98-105,scanned-probe microscopes allow an examination of a surface at veryclose range with a probe that may be just a single atom across, andresolve features and properties on a scale that eludes othermicroscopes.

The disclosure of Wickramasinghe, which is incorporated herein byreference, discloses two types of scanned-probe microscopes. The firsttype is a scanning tunneling microscope, while the second is an atomicforce microscope.

In the atomic force microscope, a scanned-probe device moves a minutetip, such as an atomically sharp diamond mounted on a metal foil over aspecimen in a raster pattern. The instrument records contours of force,the repulsion generated by the overlap of the electron cloud at the tipwith the electron clouds of surface atoms. In effect, the tip, like thestylus of a phonograph, reads the surface. The foil acts as a spring,keeping the tip pressed against the surface as it is jostled up and downby the atomic topography.

A scanning tunneling microscope senses atomic-scale topography by meansof electrons that tunnel across the gap between a probe and the surface.Piezoelectric ceramics, which change size slightly in response tochanges in applied voltage, maneuver the tungsten probe of a scanningtunneling microscope in three dimensions. A voltage is applied to thetip, and is moved toward the surface, which must be conducting orsemiconducting, until a tunneling current starts to flow. The tip isthen scanned back and forth in a raster pattern. The tunneling currenttends to vary with the topography. A feedback mechanism responds bymoving the tip up and down, following the surface relief. The tip'smovements are translated into an image of the surface.

With scanning tunneling microscopy, it is recognized that measurement ofsurface topography would be incorrect if the tip distance from thesurface is not maintained. Thus, a measurement of the force applied bythe tip on the sample throughout the measurement cycle would serve toconfirm that such distance is maintained, and provide a cross-check forthe accuracy of the topographic measurement.

As previously stated, instruments such as strain gauge transducers canbe used for micro hardness testing of samples while scanning tunnelingmicroscopes and atomic force microscopes are recognized methods formeasuring or imaging surface topography. There would be a significantadvantage when making microindentation hardness tests if it werepossible to immediately image the results with high resolutioncapability. Presently known tips and control mechanisms for scanningtunneling microscopes and atomic force microscopes have heretoforeprevented these instruments from being capable of both measuring surfacetopography and conducting microindentation hardness tests.

The tungsten scanning tunneling microscope tips generally used on theseinstruments are very slender and tend to bend into a fish hook shape atrather low indentation loads so that imaging after indentation issomewhat suspect. The atomic force microscope tips, although harder thanthe tungsten scanning tunneling microscope tips, are mounted on adelicate cantilever which is easily broken off. This limits the amountof force that can be applied with the atomic force microscope to muchless than is needed for most indentations.

An alternative approach is to build a scanning tunneling or atomic forcemicroscope with a built in scanning electron microscope which gives theimaging capability after indentation but at a considerable expense inequipment cost and added time. Also, the scanning electron microscopeonly works under vacuum so that observation of moist samples, such asbiological specimens is not possible.

In studying mechanical properties of materials on the microscopic scale,indentation and scratch testing are two frequently used techniques.Indentation testing, where a diamond tip is forced into the materialbeing tested is commonly used for determining hardness, and is beginningto be used to determine elastic modulus. The scratch test is used todetermine (among other things) the adhesion of a film or coatingdeposited on a substrate. This is done by dragging the diamond tipacross the sample surface under increasing load until a critical load isreached at which time some kind of delamination or failure occurs.

Normally the indentation or scratch is performed on one machine designedfor that purpose, and the results are analyzed by using a microscope todetermine the indent size or area of delamination. For feature sizes ofa few micrometers or greater this is usually done with an opticalmicroscope.

For features of less than a few micrometers, as are becomingincreasingly important with the continued miniaturization ofsemiconductors and decreased thickness of protective coatings, such asused on magnetic storage disks, the area would normally be determined byscanning electron microscope imaging. This involves significant work insample preparation, especially for samples that are electricalinsulators and need to be gold or carbon coated before imaging on thescanning electron microscope. Also, just finding the tiny indent orscratch is not trivial. For the smallest indents and scratches, theatomic level resolution of the scanning tunneling microscope or atomicforce microscope may be required to accurately resolve the scratchwidths and areas of delamination. Researchers have reported spending upto eight hours locating an indent on the atomic force microscope afterproducing it on a separate microindentor.

Another source of uncertainty is plastic flow or relaxation that maytake place with certain samples. If this occurs over time periods of anhour or less, an indent produced by a separate indentor may disappearbefore it can be inspected on a microscope. Indents made in the 50Angstrom range, have sometimes indicated plastic deformations that couldnot be seen with the scanning electron microscope or atomic forcemicroscope imaging. Possible explanations include mechanical hysterisisin the indentor causing it to indicate plastic deformation that was notactually present. It is also possible that there actually was an indentpresent that the researcher was not able to locate. A third possibilityis that the sample exhibited a relaxation effect where the indent wasactually present, but disappeared by some plastic flow phenomena beforethe sample could be observed in the microscope.

There would obviously be a significant advantage when makingmicroindentation hardness and scratch tests if it were possible toimmediately image the results with high resolution capability. Suchcapability would both reduce time and cost of the measurements andreduce uncertainties about the results.

Bonin et al. (U.S. Pat. No. 4,694,687) discloses a vehicle performanceanalyzer which incorporates a capacitive accelerometer for detectingchanges in G-forces and for producing a digital count value proportionalto such changes. The sensor includes a capacitive transducer comprisinga pair of spaced-apart parallel plates disposed on opposite sides of abeam-supported moveable plate, which responds to changes in accelerationof forces. Bonin et al. discloses, in FIG. 3, that the beam-supportedmoveable plate is sealed from access between the spaced-apart parallelplates. Thus, although not physically accessible, the moveable platewill yield and be displaced when subjected to G-forces duringacceleration when mounted perpendicular to such force. Bonin et al.(U.S. Pat. No. 4,694,687) is hereby incorporated by reference.

SUMMARY OF THE INVENTION

The present invention provides a force, weight or position sensor unitand sensor element in a first embodiment. In a second embodiment, thesensor element of the first embodiment is incorporated into an apparatusfor microindentation hardness testing and surface imaging which allowsimmediate imaging of the surface subsequent to hardness testing.

First, turning to the first embodiment of the present invention, aforce, weight or position sensor unit and sensor element is provided.The output from the sensor may be converted to a DC signal proportionalto the weight, force or relative position of the measured sample. Thisconversion may be accomplished as generally disclosed by Bonin et al. inU.S. Pat. No. 4,694,687, for example.

In a preferred embodiment, the sensor uses a multi-capacitor systemhaving drive and pick-up plates mounted on an appropriate suspensionsystem to provide the desired relative motion when a force is applied tothe pick-up plate. The drive plates may be driven with an AC carriersignal in the order of 50 KHz, with the driving signals being 180degrees out of phase with each other.

The output signal is run through a buffer amplifier of very high inputimpedance (100 M Ohm-0.3 pF, for example), and then synchronouslydemodulated to produce a DC signal proportional to force ordisplacement. The output is positive for one direction of displacement,and negative for the opposite direction.

A sensor element in accordance with the present invention includes apair of capacitive transducers, each transducer including a separatedrive plate and a shared pick-up plate. One of the pair of drive platesincludes a hole therethrough centrally disposed on the drive plate. Thepick-up plate is positioned between the pair of drive plates and spacedfrom each drive plate by an insulating spacer. Thus, the drive plates,in a preferred embodiment, generally include spaced opposing conductivesurfaces when the pick-up plate is mounted therebetween. The pick-upplate can be generally a conductive central plate suspended by a springmeans between the drive plates, wherein the central plate is capable ofdeflection between the conductive surfaces of each of the drive plates.

The sensor element includes means for transmitting force from a pointremote from the central plate to the central plate. The means caninclude a sample holder which is attached to the pick-up plate so thatit moves in unison with such plate. Alternatively, any rod or memberpassed through the hole in one drive plate and in contact with thecentral plate may transmit force to the pickup plate. The output isactually proportional to the pick-up plate position, but can easily becalibrated to represent force since the sensor may be constructed tohave a linear force versus displacement relationship.

In a preferred embodiment, the sample holder is a pedestal having a stemportion which passes through the centrally disposed hole in one driveplate and remains in contact with the surface of the conductive centralplate of the pick-up plate. Contact with the central plate isapproximately at its center point. Thus, the pedestal transmits a forceapplied to the pedestal to the central plate with resulting deflectionof the central plate. A diaphragm seal can be included to prevent dustor other contaminants from entering through the space between thepedestal stem and hole in the drive plate.

The disclosed force sensor is particularly useful in conjunction withscanned-probed microscopes, such as a scanning tunneling microscope oran atomic force microscope. It is, however, recognized that the sensormay be utilized in any application for measuring weight, force ordisplacement that requires high resolution of minute measurements. Theforce sensor of the present invention has a resolution of over 100,000to 1. The sensor can be of a size 1/2" square and 1/8" thick, whichallows it to be mounted on the sample holder region of an existingscanned-probe microscope. The sample to be subjected to microscopy canthen be mounted on top of the sensor. This gives a direct readout of theforce applied to the sample by the microscope tip.

The signal to noise ratio of the sensors of the present invention aremuch higher than those calculated for existing strain gauge transducers.As previously stated, the minimum possible noise level for a straingauge transducer is set by the thermal noise of the strain gaugeelement. In contrast, the capacitive sensor of the present invention hasa noise level controlled by the impedance of the sensor. This allows fora signal to noise ratio of a capacitive transducer of the presentinvention that exceeds that of a strain gauge by more than 10 times.This can be increased even further by increasing the carrier signalbeyond 50 KHz. The useable resolution is limited by thermal stability,but it is believed that the thermal stability can be improved with useof more stable materials, and that automatic correction of base linedrift is also possible.

The sensor element of the present invention comprises first and second,serially connected variable capacitors which may be readily fabricatedusing conventional printed circuit etching techniques. Morespecifically, the sensor comprises a stacked configuration of fivesubstrates.

The two outermost substrates, or first and fifth substrates, have ametalized surface on each side thereof. A portion of the metal surfaceon the inner side of the outer most plates each comprise the firstplates (drive plates) of a different variable capacitor. The firstsubstrate further includes a hole or passage therethrough for receivingmeans for transmitting force to the pickup plate (from a sample holder,for example) without contacting or being frictionally restrained frommovement therethrough. The pick-up plate is described more fully below.The fifth substrate further includes an area directly opposite andconforming to the size of the hole or passage in the first substrate inwhich the metalized surface is etched therefrom on the inner surface.This is done to maintain linearity of response of the sensor. Themetalized surfaces of the outer side of the first and fifth substratesact as shields, in known manner.

The first and fifth or outer substrates each abut the second and fourthsubstrates, respectively, which comprise insulating substrates or framemembers having an open central portion at least as large as a centralplate of the third substrate described below.

The third substrate is sandwiched between these two insulating framemembers. A portion of the third substrate comprises a common secondplate or pick-up plate for the pair of variable capacitors defined bythe first and fifth substrates. The third substrate includes a planarcentral plate which is suspended by spring-like members. In preferredembodiments, the spring-like members include four relatively thinL-shaped springs. The metal mass is thus displaceable within the frameopenings when the five substrates are sandwiched together.

The means for transmitting force to the central plate, for examplesample holder or pedestal, passes through the first and second substratewithout contact, while abutting, contacting or attaching to thesuspended metal mass proximate it center. In this way, forces applied tothe sample holder or pedestal are translated to displacement of thesuspended metal mass.

Electrical connections to various layers of substrates in theconstruction outlined above can be made by conductive pins insertedthrough metalized holes made using conventional plate through holetechniques common to multi-layered printed circuit assemblies.

Means for applying an alternating current carrier signal to the pair ofdrive plates is provided. An alternating current signal from a highfrequency oscillator is impressed across the terminals associated withthe first and fifth substrates or two outer most stationary plates ofthe transducer and the central displaceable plate (pick-up plate)provides an output. As such, a push-pull signal proportional to theamount of deflection of the central moveable plate is developed andsubsequently amplified, and then synchronously demodulated by means formonitoring an output signal. A DC voltage signal which is proportionalto force, weight or displacement can be produced.

In a second embodiment, the above described sensor can also be utilizedas a device for measuring ultra-microhardness of samples with thecapability of simultaneous or immediately subsequent scanning tunnelingmicroscopy or atomic force microscopy imaging. It has been found thatsensors of the present invention can readily provide a full scale rangeof 3 grams with resolution to 30 micrograms.

When the sensor of the present invention is utilized in an apparatus formicroindentation and imaging, the sensor is utilized to generate thedeflection signal which is presently obtained in atomic force microscopyfrom the photo sensor output of a laser reflected off the cantilever.Further, with this second embodiment, the sample is mounted on the forcesensor, and a suitable indentor tip or other hard, sharp tip is mountedon a scanning tunneling microscope piezo actuator. It has been found notnecessary for either the indentor tip or sample to be conductive, as theforce output from the sensor is sent back to the control unit, causingthe system to operate much like a standard atomic force microscope.

The sample can be imaged by specifying a contact force at a suitably lowvalue to not affect the sample. After imaging, the controller can beused to force the tip into the sample and produce the indent, with theforce sensor providing a reading of the applied load during theindenting process. The scanned probe microscope piezo can be used toforce the tip into the sample to form the indent. The sample can then bereimaged with the same tip so that the results of the indent can be seenin minutes rather than hours, as would be the case when using a separateindenting apparatus.

These and various other advantages and features of novelty whichcharacterize the present invention are pointed out with particularity inthe claims annexed hereto and forming a part hereof. However, for abetter understanding of the invention, its advantages, and the objectobtained by its use, reference should be made to the drawing which formsa further part hereof, and to the accompanying descriptive matter inwhich there are illustrated and described preferred embodiments of thepresent invention.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, in which like reference numerals indicate correspondingparts or elements of preferred embodiments of the present inventionthroughout the several views:

FIG. 1 depicts an exploded view of a capacitative sensor element inaccordance with the present invention;

FIG. 2 is a schematic representation of an apparatus for hardnesstesting and surface imaging incorporating the sensor of the presentinvention;

FIG. 2A shows a probe mounted to a force sensor which interacts with asample mounted upon a scanning head;

FIG. 2B is a third alternative embodiment showing the probe mounted onthe sensor and the sensor mounted on the scanning head;

FIG. 2C is a fourth alternative embodiment showing the probe mounted toa fixed surface and the sample mounted to the force sensor and furthermounted to the scanning head;

FIG. 3 is a photograph of an image display of the topography of a sampleprior to hardness testing on an apparatus of the present invention;

FIG. 4 is a photograph of an image display of the topography of a samplesubsequent to hardness testing on an apparatus of the present invention;and

FIG. 5 is a photograph of the image display of FIG. 4, includingsectional analysis of a sample subsequent to hardness testing.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Detailed embodiments of the present invention are described herein.However, it is to be understood that the disclosed embodiments aremerely exemplary of the present invention which may be embodied invarious systems. Therefore, specific details disclosed herein are not tobe interpreted as limiting, but rather as a basis for the claims and asa representative basis for teaching one of skill in the art to variouslypractice the invention.

The present invention includes generally two embodiments. The firstembodiment directed to a force or position indicating device or sensorand the second embodiment directed to an apparatus for microindentationhardness testing and subsequent surface imaging of the results with highresolution capacity. The second embodiment utilizes, in preferreddesigns, the sensor element of the first embodiment. The force orposition indicating device or sensor is thus described first. Theapparatus for microhardness testing and subsequent surface imagingutilizing the sensor is then described, recognizing that the disclosurewith regard to the force sensor alone is equally applicable to the testapparatus utilizing such sensor.

The force (including weight) or position indicating device or sensor ofthe present invention generally has three components. The firstcomponent is a sensor element, which is a multi-plate capacitor system.A second component is means for inputting an AC carrier signal, whilethe third component is means for monitoring the sensor element output,preferably converting the output from the sensor to a DC signalproportional to force, weight or displacement.

Referring now to FIG. 1, an exploded view of the components of thesensor element 2 of the present invention, is depicted. Functionally,the sensor element comprises two transducers 4, 6, which function as twovariable capacitors connected in series and forming a capacitive voltagedivider. The overall sensor element 2 includes five substrate layers 8,10, 16, 14, 12 sandwiched together to form the transducers. The sensorelement 2 can be fabricated using well-known printed circuit etchingtechnology.

The first substrate layer 8 and the fifth substrate layer 12 include thedrive plates or fixed plates of the transducers and are driven with acarrier signal. The carrier signal can be an AC signal on the order of50 KHz, with the signal to these outer most substrate layers 8, 12,being 180 degrees out of phase with each other.

The outer exposed surfaces of first substrate 8 and fifth substrate 12are covered with metalization, for example, copper. This metal layerfunctions as a shield against EMI noise. On the inner surface of firstsubstrate 8 and fifth substrate 12, a metalized pattern 30 is provided.This metalized pattern forms the drive plate on each substrate. Themetalized pattern on the interior surface of the first substrate 8generally corresponds to that on the fifth substrate 12. As depicted inFIG. 1, the metalized pattern 30 or drive plate on the inside of thefifth substrate 12 can include a generally rectangular frame pattern 31.Extending around the periphery of the substrate metalized pattern 31 isa channel defining an unmetalized opening 32. Centrally disposed in thisunmetalized opening 32 is the rectangular metalized pattern 31 ofconductive material, having a conductive lead 33 leading to a conductiveterminal portion 34.

The metalization on the inside surface of the first substrate 8 issimilar to that of the inside surface of fifth substrate 12 with twoexceptions. The first difference is that the terminal portions of eachsubstrate 34, 36 are offset from one another, rather than beingvertically aligned when the sensor element 2 is assembled. The seconddifference is the provision of a through hole 22 centrally disposedthrough the thickness of the first substrate 8. The through hole 22 isdisposed centrally for receiving a sample holder 24 or other means fortransmitting force therethrough, which is described in further detailbelow.

The inside surface of the fifth substrate 12 includes a demetalized oretched portion 38 which corresponds to the through hole 22. Theprovision of the demetalized or etched portion 38 generallycorresponding to the through hole 22 provides for the rectangularmetalized pattern 30 of conductive material on each of the firstsubstrate 8 and fifth substrate 12 inside layers to be mirror images ofone another. This provision is necessary to provide a linear responsefrom the pair of capacitive transducers 4, 6.

The outer layers of the sensor element 2 or first substrate 8 and fifthsubstrate 12 can be manufactured from standard circuit board materials,such as 1/16" glass epoxy with copper on both sides. In order to reducelabor requirements, a large number of the outer layer substrates may bemanufactured at one time. For example, a 6" sheet of material may beutilized to manufacture about 100 substrate layers of 1/2" squaredimensions. The pattern for the metalized portion 30 of the firstsubstrate 8 and fifth substrate 12 may be first etched in the copper.The substrate may be routed around the individual devices within a largesheet of material, leaving only thin tabs of materials to hold themtogether. These tabs allow the devices to be snapped apart afterassembly.

The second substrate layer 10 and the fourth substrate layer 14 comprisespacer layers. As depicted in the figure, these layers 10, 14 may be ofgenerally rectangular shape and have a generally rectangular openingformed centrally therein, with the opening extending completely throughthe substrate. The spacer layers, second substrate 10 and fourthsubstrate 14, must be insulators or covered with an insulating coating.The opening through the insulators 10, 14 is equal to or greater thanthe dimensions of a central plate 20 on a third substrate 16 describedbelow, and an associated appropriate suspension system 18, alsodescribed below.

The second substrate 10 and fourth substrate 14 can be manufactured frometched metal with an insulating coating on both sides. This insulatingcoating could be an epoxy, or other organic coating such as those usedon enameled magnet wire, but it is believed that best results areachieved by using aluminum for the spacer and anodizing it to form aninsulating coating of aluminum oxide.

It is believed that the insulating spacers, second substrate layer 10and fourth substrate layer 14, can be etched first and then anodized, oranodized first and then etched, depending upon the type of photoresistchemicals used. A preferred method is to use aluminum sheet stockpurchased with a thin (0.00012") anodized layer on both sides. Thisanodized layer provides good adhesion with the positive type liquidphotoresist which can be used to fabricate the other layers of thesensor element 2. With bare aluminum, the resist tends to peel away atthe edges being etched making it hard to maintain desired dimensions.

After etching, the photoresist and original anodizing are removed andthe parts are anodized to the desired insulation thickness. Although itis believed 0.0005" or less of an anodized thickness layer will providethe required electrical isolation, it is desirable to make the thicknessas great as possible to minimize the capacitance between the outer layershields, first substrate layer 8 and fifth substrate layer 12, and acenter plate, third substrate layer 16, described below.

The third substrate layer 16 is sandwiched between the insulatinglayers, second substrate layer 10 and fourth substrate layer 14. Thethird substrate layer 16 includes the pick-up plate which is common tothe pair of transducers 4, 6. A central plate 20 is mounted on anappropriate suspension system 18 to provide for desired relative motionof the central plate 20 or pick-up plate on third substrate layer 16.The third or central substrate layer 16 can be an etched metal layersupported by a suspension system 18 defined by a pattern of slits 19.The central plate 20 is thus a solid portion or mass suspended by thesurrounding framework of a suspension system 18. The third substratelayer further includes a terminal 17 for electrical connection. Apreferred metal for use as a central plate is a beryllium-copper alloy.

Although a pattern of four L-shaped slits 19 are depicted in the figure,it is believed that other patterns may be utilized to provide the sametype of spring supporting structure for central plate 20. Further, it isrecognized that varying effective spring constants may be achieved forthe centrally supported mass or central plate 20 by altering thethickness of the materials of this substrate and the size of the springelements. Thus, the overall range of travel per unit force exerted onthe central plate 20 of the third substrate layer 16 may be varied bydesign. Thus, sensors of varying overall range may be manufactured.

When the five substrate layers 8, 10, 16, 14, and 12 are assembledtogether, the central plate 20 of the third substrate layer 16 iscentrally disposed within the openings formed in the insulatingsubstrates, second substrate layer 10 and fourth substrate layer 14, andthus, the central plate 20 is free to deflect relative to the firstsubstrate layer 8 and fifth substrate layer 12.

The layers may be assembled together by hand, holding them together withpins inserted around the entire perimeter of the substrates and solderedto the outside layers. When assembled, selected electrical connectionsbetween the various internal layers or substrates can readily beprovided as disclosed by Bonin et al. in U.S. Pat. No. 4,694,687.

Means for transmitting force 24 from a point remote from the centralplate 20 to the central plate 20 are provided. This means can include asample holder 24, which functions to transmit the force created by theweight of a sample to the central plate 20 of the third substrate layer16. In a preferred embodiment, the sample holder 24 is a pedestal whichincludes a sample holding surface 26 and a stem portion 28. The stemportion 28 extends through the through hole 22 in the first substratelayer 8 and through the opening in the second substrate layer 10. Thebottom surface 29 of the stem portion 28 contacts the upper surface ofthe central plate 20 at a central point 23 when the sensor is assembled.The space between the stem portion 28 and wall of the through hole 22 ispreferably sealed from contamination by a diaphragm seal or othersealing means which prevents entry of dirt while not impeding movementof the pedestal or other means for transmitting force 24.

Thus, functionally, the weight or force exerted by a sample or othermeans on the sample holding surface 26 of the sample holder 24 istransmitted to the central plate 20 of the third substrate layer 16 andresults in deflection of the central plate 20 commensurate with theforce exerted on the surface of the sample holder 24. Thus, the centralplate 20, under force, moves closer toward or further away from one orthe other of the outer most substrates, first substrate layer 8 andfifth substrate layer 12. Of course, the sample holder 24 may bedirectly connected to a moving, or force imparting, element withoutpositioning a "load" on the surface 26. Indeed, the surface 26 may bereplaced by a connector adapted for this purpose.

Means for providing a carrier signal to the outer most plates or firstsubstrate layer 8 and fifth substrate layer 12 are provided. This signalcan be an AC signal. Such means may include an oscillator which producesa 50 KHz alternating current signal. The signal to each outer most plateis preferably 180 degrees out of phase with the signal provided to theother outer most plate.

Means are also provided for reading the output from the sensor element2, and converting the output to a signal proportional to force, weightor displacement of the central plate 20. The output signal is generallyrun through a buffer amplifier of very high input impedance (100MOBM-0.3 pF), and then synchronously demodulated to produce a DC signal.The DC signal is proportional to the force, weight or displacement ofthe central plate 20. The output would be positive for one direction ofdisplacement, and negative for the opposite direction. It is recognizedthat the sample holder 24 or means for transmitting force is attached orin contact with the central plate 20 to move in unison with such centralplate 20. The output of the sensor 2 is actually proportional to thecentral plate 20 position, but can easily be calibrated to representforce (including weight) since the sensor has a linear force versusdisplacement relationship.

It is recognized that the sample holder 24 or means for transmittingforce must be manufactured from an insulating material or covered withan insulating material. Further, the clearance between the insidediameter of through hole 22 and the outside diameter of stem portion 28must be sufficient to avoid any frictional effects which may reduce thesensitivity of the sensor element 2.

The signal to noise ratio of the capacitive transducers of the presentinvention are much better than that of presently used metal strain gaugetransducers. The minimum noise level of the strain gauge transducer isdetermined by the thermal noise of the strain gauge resistive element.This noise is proportional to the square root of the resistance. Theoutput signal is proportional to the input signal, but is only a verysmall fraction of it. A typical value taken from a commercial scalestrain gauge transducer is 175 Ohm resistance at full scale output of 5millivolts.

The three-plate capacitive transducer of the present invention does notgenerate noise as a resistive transducer does, but the signal cannot beused without connecting it to an amplifier, and the amplifier must havea very high input resistance, so the amplifier will generate noise. Thelower limit of this noise will be determined by the effective inputimpedance of the amplifier. Since the capacitive transducer is inparallel with the amplifier input impedance, and the amplifier inputimpedance is much larger than the impedance of the transducer (or theoutput will be very non-linear), the effective input impedance is equalto that of the transducer.

The impedance of the transducer is determined by the capacitance andoperating frequency. Higher operating frequency gives lower transducerimpedance (X_(c) =1/6.28 FC). The capacitance is about 10 pF for the1/2" square device with 0.005" spacing between plates. The operatingfrequency can be any convenient value, limited only by the frequencyresponse of the amplifier and associated circuitry. The full scaleoutput signal of the transducer is equal to the input voltage, whichwill be conservatively taken as 10 volts. The full scale output of thecapacitive transducer is 10V, which is 2,000 times greater than thestrain gauge transducer (5 mV). The impedance, and therefore the noisegenerated, is greater with the capacitive transducer (except at veryhigh frequencies which would require rather expensive components), butdue to the much higher inherent output level, the signal to noise ratioof the capacitive transducer is significantly better.

The following table shows the relationship of signal to noise ratio forthe two transducers.

                  TABLE 1                                                         ______________________________________                                        Fop = operating frequency of capacitive transducer                            C = capacitance of transducer = 10 pF                                         Xc = impedance of transducer = 1/(6.28 × Fop × C)                 R = resistance of strain gauge = 175 Ohm                                      Since noise is proportional to the square root of R or Xc, the ratio of       capacitive transducer noise to strain gauge noise is the square root of       (Xc/R), and the factor of improvement of SNR of capacitive vs strain          gauge is 2000 divided by the square root of (Xc/R).                                              square root                                                Fop            Xc/R                                                                                       Xc/R                                                                                    2000/sq root (Xc/R)                     ______________________________________                                         10 KHz   11,400   107          19                                            100 KHz      1,140                            59                               1 MHz         114                           190                               10 MHz         11.4                                                                                         3.4                                                                                       590                                100 MHz          1.14                                                                                       1.1                                                                                       1900                                                                            Capacitive                                                                 transducer SNR                                                                is better than                                                                strain gauge by                                                               factor in above                                                               column.                              ______________________________________                                    

As is readily apparent from the above table, the capacitive transducersensor of the present invention is far superior to strain gauges on thebasis of electronic noise.

Since the output of the capacitive transducer or sensor element 2 isproportional to the displacement of the center mass portion 20 orelectrode, it is recognized that a device for use as a scale or as ameasure of displacement may be manufactured. It is first necessary tochoose an appropriate stiffness for the suspension system 18 supportingthe central plate 20 so that the sample holder 24 or means fortransmitting force is forced reliably against the surface to be measuredwithout exerting excessive force that would deflect the object andchange its actual position. Secondly, it is recognized that theinsulating spacers, second substrate layer 10 and fourth substrate layer14, may be manufactured of different thicknesses to offset the centerplate sufficiently. This would alter the operational range of thedevice. With resolution of one part in 100,000, it is believed that suchsensors can resolve displacements down to 0.1 microinches or just 25Angstroms or better.

Now referring to FIG. 2, a schematic representation of an apparatus forhardness testing and surface imaging incorporating the above-describedsensor of the present invention is depicted. With this secondembodiment, it is possible to conduct a scan of the surface topographyof a sample, followed immediately by microindentation testing, followedby a second imaging of the surface topography all on the sameinstrument. Generally, the schematic in FIG. 2 depicts a commercialscanning tunneling microscope, such as the Nanoscope III, available fromDigital Instruments, which has been modified to conduct the in-situ highresolution imaging and microindentation testing on a single instrument.

As previously stated, scanning tunneling microscopes are commerciallyknown. As disclosed by Wickramasinghe in "Scan-Probe Microscopes",Scientific American, October, 1989, pp. 98-105, which is incorporatedherein by reference, scanning tunneling microscopes include severalstandard components which are depicted in FIG. 2.

With a scanning tunneling microscope, a sample 52 is placed on a sampleplatform 54 for analysis. The scanning tunneling microscope sensesatomic-scale topography by means of electrons that tunnel across the gapbetween a probe 50 and the surface of the sample 52. A scanning head 58,(a piezo actuated head in the illustrated embodiment) has the probemounted thereon. In other embodiments, the scanning head 58 may includea 3-D piezo actuator. The scanning head 58 is utilized to move the probein three directions in response to changes in applied voltage.Piezoelectric ceramics are generally utilized because they change sizeslightly in response to such changes in voltage, and thus, maneuver theprobe in three dimensions. The voltage applied to the scanning head 58is controlled by the scanning tunneling microscope controller 60.

In use, voltage is applied to the tip of the probe 50 and it is movedtoward the surface of the sample 52, which must be conducting orsemiconducting, until a tunneling current starts to flow. The tip of theprobe 50 is then scanned back and forth in a raster pattern by varyingthe voltage to the piezoelectric ceramics which control horizontalmotion. The tunneling current tends to vary with the topography of thesample, and therefore, a current output signal 66, which provides afeedback mechanism, and which monitors such tunneling voltage, feedssuch signal to the scanning tunneling microscope controller 60. Thecontroller 60 adjusts the output to the scanning head 58 which respondsby moving the tip of the probe 50 up and down, following the surfacerelief. The probe's 50 movements are translated into an image of thesurface and displayed on an image display 62.

With scanning tunneling microscopy, the probe 50 is generally made fromtungsten with a tip so fine that it may consist of only a single atomand measures as little as 0.2 nanometers in width.

The apparatus of Applicant's present invention for microindentation withsubsequent surface imaging utilizes the above-described scanningtunneling microscope with several modifications. A force sensor 56, asdescribed in the first embodiment, is mounted on the scanning tunnelingmicroscope base in place of the standard sample holder. The sample 52 isthen mounted on the sample platform 54. A force controller 64 isoperatively connected to the force sensor 56 to monitor the outputsignal from the force sensor 56 and convert it to a signal proportionalto the force being applied to the sample 52 on the platform 54 by theprobe 50. The force controller 64 or the STM controller 60 can eitherseparately or together perform functions of the means for providing acarrier signal 40 and the means for reading and converting the output 42(shown in FIG. 1). The force controller or force sensor output signalmay then be utilized to control the vertical position of the probe 50 orposition along the Z axis by sending such signal through the scanningtunneling microscope controller 60 during surface imaging.Alternatively, the output from the force controller 64 can be monitoredfor measurement of force being applied during micro-indentation or microhardness testing. These procedures are described below.

The scanning tunneling microscope described above is also modified byreplacing the tungsten probe with a harder tip for microindentationtesting. In a preferred embodiment, a diamond tip is used, such as bluediamond. It is not necessary for the tip to be conductive or a samplebeing tested to be conductive; however, it is recognized that conductiveblue diamond scanning tunneling microscope tips are available. They canbe used for scanning tunneling microscopy imaging of conductive samples,as well as testing with Applicant's apparatus.

In operation, the force sensor 56 of Applicant's second embodiment isused for both measuring the applied force during indentation orscratching and for imaging before and After testing. An atomic forcemicroscope type image is first obtained from the scanning tunnelingmicroscope by disconnecting the scanning tunneling microscope'stunneling current output signal 66 and substituting in its place theoutput signal 68 from the force sensor 56. The scanning tunnelingmicroscopes scanning function can then be operated in a normal manner,with the force controller 64 output signal now controlling the Z axispiezoceramic to maintain a constant force between the probe 50 tip andthe sample 52, rather than a constant tunneling current. Alternatively,a constant height image could be obtained where the probe 50 tipZ-position or vertical height is held constant, and the image isobtained directly from the force sensor 56 output signal from the forcecontroller 64, which again passes through the scanning tunnelingmicroscope controller 60 and results in a display of surface topographyon the image display 62.

Once an image of the surface has been made using the above procedure,the controller can be used to force the tip into the sample and producean indent, with the force sensor providing a reading of the applied loadduring the indenting process. The scanned probe microscope piezo can beused to force the tip into the sample to form the indent. In particular,in a preferred embodiment, the Z axis piezo can be manipulated toprovide force to the tip which provides an indentation. Afterindentation, the sample can then be reimaged with the same tip so thatthe result of the indent can be seen in minutes, rather than hours,without the need for moving the sample or finding the point where theindentation was made in the sample. Further, because the first image,indentation, and second image are all made with the sample in a singleposition, it is assured that the first surface image and second surfaceimage are of the same surface area and show the corresponding effect ofthe indentation step.

With the above-described system, both conducting and non-conductingsamples can be imaged at high resolution before and after mechanicaltesting without disturbing the sample position so that there is noproblem of trying to locate the test region as there is when usingseparate indenting and imaging equipment. It is also possible to compareside by side atomic force microscope images and scanning tunnelingmicroscope images of the same sample surface by flipping a switch tochange from atomic force microscope to scanning tunneling microscope.This is sometimes useful as the atomic force microscope signal isgenerally an accurate representation of the sample topography, while thescanning tunneling microscope signal may give some information aboutconductivity or electronic states of the surface.

Atomic force microscope images have been obtained using the sensor andmicroscope apparatus of the present invention, as described above.Photographs of images derived from the above apparatus are included inFIGS. 3, 4, and 5, and reference should be made thereto. The microscopeutilized was a Digital Instruments Nanoscope III using a blue diamondprobe. The sample utilized was a GaAs semiconductor wafer that waspolished prior to testing. The force sensor scale factor was 5.59V/gram, and the scanning tunneling microscope set point was 200 pA.

The image of FIG. 3 was taken before indenting while the image in FIG. 4was taken after applying a force of 77.8 mg. The surface topography, asaffected by the indentation, is evident from the recorded imagevariation in color. The rust to dark brown area clearly showing thedeepest indentation. The third image or FIG. 5 is a sectional analysisof the indent of FIG. 4 at its deepest section. The relevant data is thehorizontal distance in red of 0.383 micrometers and the verticaldistance in green of 0.0497 micrometers.

The operating force during imaging is determined to be the combinationof the sensor scale factors, the current to voltage conversion factorfor the microscope, and the tunneling current setpoint. For the abovescanning tunneling microscope, the factor was 0.1 V/nA so with the setpoint of 200 pA, the instrument would apply whatever force was requiredto produce 20 millivolts which is the equivalent of 3.58 milligramsforce. Due to a 5 millivolt offset error somewhere in the system in theabove tests, the 200 pA setpoint generated a 15 mV, rather than 20 mVfor sensor output and the force sensor had an output of 10 mV at 0 load,so the actual load was about 0.89 mg. These offsets can be correctedwith adjustments to the setpoint of the scanning tunneling microscope.

The apparatus for microindentation hardness testing of surface imagingof the present invention has been described with respect to a preferredembodiment in which a scanning tunneling microscope apparatus isutilized having a base for mounting a sample thereon and a piezoactuated head having a probe mounted thereon for operative engagement ofa sample mounted on the base for measuring surface typography. In thisembodiment, a probe is mounted on the piezo actuated head, while theforce sensor is mounted on the base for mounting a sample thereon. Withthis arrangement, the scanning head or piezo actuated head moves theprobe in a raster pattern over the surface dimension typography. It is,however, recognized that other arrangements of the probe, force sensorand scanning head are possible within the scope of the presentinvention. The key to operation of applicant's invention is that ascanned probe microscope apparatus incorporates a probe in a scanninghead arranged for operative engagement of a surface of a sample formeasuring a surface typography thereof. The probe has a hardness greaterthan a sample to be tested and the force sensor is operatively locatedto measure the force between the sample and the probe when operativelyengaged in the surface thereof.

As previously stated, in a first preferred embodiment, scanned probemicroscope includes a base for mounting a sample thereon and a piezoactuated head having a probe mounted thereon, with the force sensormounted on the base 5 and the sample resting thereon. In a secondpreferred embodiment, as shown in FIG. 2A, the force sensor may bemounted on a fixed surface with the probe affixed to the force sensor.The sample may be mounted on a sample holder which incorporates a piezoactuated head or scanning head. With this arrangement, the piezoactuated scanning head moves the sample against the probe with the forceapplied to the probe translated through the sensor to measure the force.

In a third alternative embodiment, as shown in FIG. 2B, the samplehaving a surface to be scanned may be a large sample on which aninstrument of the present invention may be mounted. The instrument wouldinclude the probe mounted on the force sensor, which in turn is mountedon the piezo actuated or scanning head. With this arrangement, the probeis placed to engage the surface of the large sample and the force sensoris again utilized to measure the force of contact, while the scanninghead moves the probe over the surface for imaging.

In a fourth alternative embodiment, as shown in FIG. 2C, the probe canbe mounted on a fixed surface. With this arrangement, the sample andforce sensor are mounted on the piezo actuated or scanning head. Thus,the scanning head moves the sample over the fixed probe with the sensormeasuring the force between such probe and sample.

New characteristics and advantages of the invention covered by thisdocument have been set forth in the foregoing description. It will beunderstood, however, that this disclosure is, in many ways, onlyillustrative. Changes may be made in details, particularly in matters ofshape, size, and arrangement of parts, without exceeding the scope ofthe invention. The scope of the invention is, of course, defined in thelanguage in which the appended claims are expressed.

What is claimed:
 1. In a scanned probe microscope apparatus having aprobe and a scanning head operably arranged for measuring surfaceproperties of a sample, the apparatus improvement comprising:a highprecision capacitance sensor having a pick-up plate movably mountedrelative to a drive plate; means for transmitting force between anobject remote from the pick-up plate and the pick-up plate; and meansresponsive to the position of the pick-up plate relative to the driveplate for providing an output signal proportional to the relativeposition.
 2. The apparatus of claim 1, wherein the means fortransmitting force between an object remote from the pick-up plate andthe pick-up plate includes the pick-up plate mechanically linked to theprobe.
 3. The apparatus of claim 1, wherein the means responsive to theposition of the pick-up plate relative to the drive plate includes meansfor utilizing said output signal to control a vertical movement of thescanning head relative to the sample.
 4. The apparatus of claim 3,wherein the vertical movement of the scanning head relative to thesample is controlled to maintain a constant force on the sample as thesurface property is measured.
 5. The apparatus of claim 1, wherein themeans responsive to the position of the pick-up plate relative to thedrive plate includes means for applying an AC signal to the drive plate.6. The apparatus of claim 1, wherein the means responsive to theposition of the pick-up plate relative to the drive plate synchronouslydemodulates the output signal to produce a DC signal proportional to thedisplacement of the pick-up plate.
 7. In a scanned probe microscopeapparatus having a probe and a scanning head arranged for operativeengagement of the surface of a sample for measuring a surface propertythereof, the apparatus improvement comprising:a force sensor operativelylocated to measure the surface property, the force sensor having anoutput signal representative of the measured surface property, whereinthe force sensor includes:a pair of capacitive transducers, eachtransducer including a separate drive plate and a shared pick-up platemovably suspended between the drive plates, wherein said pick-up plateis capable of deflection between each of the drive plates; and means fortransmitting force from a point remote from the pick-up plate to thepick-up plate.
 8. The apparatus of claim 7, further comprising means formeasuring the output signal of the force sensor and utilizing the outputsignal to control a vertical movement of the scanning head.
 9. Theapparatus of claim 8, wherein the vertical movement of the scanning headis controlled to maintain a constant force on the sample as the surfacetopography is measured.
 10. The apparatus of claim 8, further comprisingmeans for applying a downward force to said probe, wherein said forcesensor measures the force and the means for measuring the output signalof the force sensor converts the output signal to a signalrepresentative of the force during an indentation test.
 11. Theapparatus of claim 7, further comprising means responsive to the outputsignal for controlling the movement of the scanning head.
 12. Theapparatus of claim 11, wherein the movement of the scanning head iscontrolled in a vertical direction.
 13. The apparatus of claim 12,wherein the means responsive to the output signal further controls themovement of the scanning head in a two-dimensional horizontal direction.14. The apparatus of claim 13, wherein the means for controllingmovement of the scanning head provides an output signal to an imagedisplay, wherein the image display provides an image representative ofthe surface property being measured.
 15. The apparatus of claim 7,further comprising means responsive to the output signal for providingan image representative of the surface topography.
 16. In a highresolution sensor apparatus for use with a scanned-probe microscopehaving a probe and a scanning head operably arranged for measuringsurface properties of a sample, the apparatus improvement comprising:a.a sensor element, said sensor element including,1. a pair of capacitivetransducers, each transducer including a separate drive plate, and ashared pickup plate, said pickup plate positioned between said separatedrive plates, said drive plates having spaced opposing conductivesurfaces when said pickup plate is mounted therebetween, said pickupplate further comprising a conductive central plate suspended by springmeans between said drive plates, wherein said central plate is capableof deflection between the conductive surfaces of each of said driveplates;
 2. means for mechanically transmitting force from a point remotefrom said central plate to said central plate; and b. means forproviding an output signal representative of the surface property beingmeasured, including means for applying an alternating current carriersignal to said pair of drive plates, wherein the signal to one of saiddrive plates is 180 degrees out of phase with the signal to the other ofsaid drive plates.
 17. The apparatus of claim 16, further comprisingmeans for measuring the output signal of the sensor element andutilizing the output signal to control a vertical movement of thescanning head.
 18. The apparatus of claim 17, wherein the verticalmovement of the scanning head is controlled to maintain a constant forceon the sample as the surface topography is measured.
 19. The apparatusof claim 16, further comprising means for applying a downward force tosaid probe, wherein said sensor element measures the force and the meansfor measuring the output signal of the sensor element converts theoutput signal to a signal representative of the force during anindentation test.
 20. The apparatus of claim 16, further comprisingmeans responsive to the output signal for controlling the movement ofthe scanning head.
 21. The apparatus of claim 20, wherein the movementof the scanning head is controlled in a vertical direction.
 22. Theapparatus of claim 21, wherein the means responsive to the output signalfurther controls the movement of the scanning head in a two-dimensionalhorizontal direction.
 23. The apparatus of claim 22, wherein the meansfor controlling movement of the scanning head provides an output signalto an image display, wherein the image display provides an imagerepresentative of the surface property being measured.
 24. The apparatusof claim 16, further comprising means responsive to the output signalfor providing an image representative of the surface topography.